While not limited thereto, the present invention is particularly adapted for use with basic oxygen furnaces utilized in the steel industry. In a typical BOF of this type, the end of tapping (i.e., pouring molten metal from the vessel) is characterized by the level metal at the tap hole being below a critical height at which the slag above the metal begins to vortex through the metal and occupies the core of the molten metal stream coming out. This can be compared to water draining from a sink. As the water level falls, a point is reached where a vortex occurs over the drain; and water from the surface is sucked down into the vortex. In order to tap clean steel (i.e., steel with no slag entrapment), the tapping procedure must be stopped using present-day prior art techniques when approximately 10% of the total steel is still in the BOF.
The tap hole of a typical BOF has its opening facing the slag-metal interface. Detection of slag in the metal stream is not easy since it is usually in the core of the stream and cannot be seen.
In the past, various techniques have been used to prevent vortexing of the type described above. For example, electro-magnetic devices have been devised which detect the start of slag vortexing. These devices are very reliable, but unfortunately they are also very expensive and in many cases they do not justify their implementation from the point of view of cost. Cheaper methods such as using a refractory cube, which floats at the slag-metal interface and delays the start of slag vortexing, are not altogether reliable as are pneumatic devices. In short, existing methods of tapping slag-free steels are either very expensive or unreliable, with the result that vortexing of the slag has continued to be a bottleneck in obtaining the maximum obtainable yield of clean steel from the BOF.